Extensile to contractile transition in active microtubule-actin composites generates layered asters with programmable lifetimes

Abstract: We study a reconstituted composite system consisting of an active microtubule network interdigitated with a passive network of entangled F-actin filaments. Increasing viscoelasticity of the F-actin network controls the emergent dynamics, inducing a transition from turbulent-like flows to bulk contractions. At intermediate F-actin concentrations, where the active stresses change their symmetry from anisotropic extensile to isotropic contracting, the composite separates into layered asters that coexist with the background turbulent fluid. Contracted onion-like asters have a radially extending microtubule-rich cortex that envelops alternating layers of microtubules and F-actin. The self-regulating layered organization survives aster merging events, which are reminiscent of droplet coalescence, and suggest the presence of effective surface tension. Finally, the layered asters are metastable structures. Their lifetime, which ranges from minutes to hours, is encoded in the material properties of the composite. Taken together, these results challenge the current models of active matter. They demonstrate that the self-organized dynamical states and patterns, which are evocative of those observed in the cytoskeleton, do not require precise biochemical regulation but can arise due to purely mechanical interactions of actively driven filamentous materials.